A novel methodology for modeling dwell fatigue crack growth in Ni-based superalloys

A novel, empirically-based damage tolerance life prediction methodology has been developed to model dwell fatigue crack growth (DFCG) behavior in Ni-based disk superalloys. The methodology is based on a new crack driving force parameter, termed Ksrf, which was developed specifically to account for dwell crack growth. The approach taken in the development of this parameter was to account for the apparent contradiction between the brittle nature of the oxide induced intergranular crack growth process and the extensive creep deformation occurring simultaneously in the crack tip region's visco-plastic zone. It was hypothesized that the rate of the brittle intergranular crack growth is controlled by the magnitude of the crack tip tensile stress field which varies depending on the constitutive visco-plastic behavior of the various microstructures evaluated. The new parameter accounts for these differences by measuring and normalizing the remaining tensile stresses generated by simulations of the crack tip visco-plastic behavior through simple stress relaxation tests. The Ksrf parameter was able to account for up to an order of magnitude of differences in DFCG rates in the LSHR and RR1000 alloys when correlated using standard linear elastic fracture mechanics (LEFM) K stress intensity parameter. The same Ksrf methodology was able to explain and account for the substantial effects that small overloads have on DFCG behavior.